This invention relates to a failure detecting circuit for gate-turn-off thyristors that detects short-circuit failures of gate-turn-off thyristors at the gate circuits thereof.
Hitherto, a short-circuit failure of a gate-turn-off thyristor (hereinafter "GTO thyristor") has been determined by detecting a short-circuit current or a voltage drop between the anode and cathode of the GTO thyristor. However, plural GTO thyristors often are connected to each other in series. If one of the GTO thyristors breaks down, it does not cause a short-circuit current to flow. Accordingly, the voltage across the assembly of serially connected GTO thyristors does not become low.
In order to detect a failure in serially connected GTO thyristors, a circuit as shown in FIG. 1 has been utilized. In FIG. 1, reference numerals 11 through 16 represent GTO thyristors. In this case, six GTO thyristors are connected in series. Reference numerals 17 and 18 represent voltage-dividing resistors, and 19 a current detector, respectively. When all the GTO thyristors 11 through 16 are in normal operation, a voltage between points C and B is half of a voltage between points A and B. Similarly, a voltage between points D and B is half of the voltage between points A and B. In this event, the potentials at points C and D are equivalent, so that there exists no current flow between points C and D.
When a GTO thyristor in the group 11, 12, 13, such as GTO thyristor 11, experiences a short-circuit failure, the voltage between points C and B rises to greater than half of the voltage between points A and B. Consequently, a current flows from point C to point D. A current detector 19 is connected between points C and D. If current detector 19 detects the current that flows from point C to point D, a GTO thyristor in the group 11, 12, 13 is judged as having failed. On the other hand, when a GTO thyristor in the group 14, 15, 16, such as GTO thyristor 14, experiences a short-circuit failure, a current flows from point D to point C. If current detector 19 detects this current, the failure of a GTO thyristor in group 14, 15, 16 is determined.
In the aforementioned conventional method, a failure of a GTO thyristor element is detected by the difference between the voltage across points A and C and that across points C and B. In this method, there is a disadvantage described below. When the number of the GTO thyristors between points A and C in short-circuit failure is equal to that of the GTO thyristors between points C and B in short-circuit failure, the failure of such GTO thyristors cannot be detected, in principle.
Furthermore, as the number of serially connected GTO thyristors becomes large, resistors 17, 18 which will withstand higher voltages are inevitably required.